Science · 6 pages · 12 min
How yeast makes bread rise
Yeast turns sugar into carbon dioxide, and the dough traps the gas.
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From the opening
Yeast makes bread rise by feeding on sugars in the dough and releasing carbon dioxide. The gluten network traps those bubbles, so the loaf expands instead of letting the gas escape.
The rise is not air that was beaten in and then simply warmed. Kneading folds in a little air, but the large increase during proofing is new gas made by living yeast.
The mechanism is more particular than the slogan. Yeast cells run a fermentation that breaks sugar down without needing oxygen. Carbon dioxide is one product, and ethanol is the other, which mostly leaves the loaf later in the oven.
Follow it one step further. Heat in the oven expands the gas already trapped in the dough and sets the starch and protein so the bubbles stay. If the structure cannot hold, the loaf collapses instead of keeping the height the gas earned.
The case worth keeping in mind is specific. A plain dough of flour, water, salt, and yeast left in a warm place grows as the cells work. The same dough kept cold grows much more slowly, because the metabolism is slower, not because the recipe changed.
That case has a boundary. If the flour has little gluten, as in many cakes, the gas escapes or the crumb stays weak and the rise is small. Chemical leaveners such as baking soda make gas by a reaction, not by fermentation, so they only share the visible result.
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Inside
- Fermentation makes the gas
- Gluten traps the bubbles
- What the oven then does
- How this differs from baking soda
- Where the idea stops applying
- A distinction worth keeping
Sources
- Alcoholic fermentation by baker's yeast
- The gluten network that traps carbon dioxide